The present invention relates to nuclear reactor fuel assemblies, and more particularly, to grids which carry dedicated structure for enhancing coolant mixing in the fuel assembly.
U.S. Pat. No. 4,879,090, "Split Vaned Nuclear Fuel Assembly Grid", issued Nov. 7, 1989 to Patrick A. Perrotti et al, discloses a nuclear fuel assembly grid in which mixing vanes project from the upper edges of the constituent strips, so as to induce a so-called "alternating swirl" flow pattern between adjacent cells of the grid. In other words, when viewed in plan, the swirl pattern induced by the vanes in one cell follows a clock-wise direction, whereas the swirl flow pattern in an adjacent cell follows a counter-clock-wise pattern, such that the swirling flow midway between adjacent fuel rods, is generally in the same direction.
Such alternating swirl patterns have been shown to be very effective at improving heat transfer in a nuclear fuel assembly, relative to other mixing flow patterns that can be induced by dedicated mixing vane structure or the like. There are, however, at least two drawbacks to grids which have the split vane configuration such as shown in said patent. First, the cantilevered, free-standing vanes which project from the upper edges of the strips, are susceptible to bending damage during rod insertion for both initial fabrication and reconstitution. Secondly, free standing vanes suffer a loss of efficiency in flow redirection due to their relatively narrow width and the leakage losses at the open sides of the vanes.
Another disadvantage in some circumstances, is the incompatibility of the split vanes as formed on the flat strips from which the grid is constituted as shown in said patent, if incorporated into the "wavy strip", or "corrugated strip" grids of the type disclosed, for example, in U.S. Pat. No. 4,897,241 issued Jan. 30, 1990 to Anthony. The problem encountered is illustrated by FIGS. 1 and 2 which accompany the present specification. As shown in FIG. 1, if the cell walls are straight, they form squares having 90 degree corners. The vanes such as 10 and 12 which project from the upper edges of opposite walls 14, 16 of cell 18, have a straight edge 20,22 which extends perpendicularly from the upper edge of the given wall, and parallel to the adjacent wall 24, 26.
In the wavy strip type of grid the cells 28 are not rectangular. Rather, two adjacent convex walls 30,32 and two adjacent concave walls 34, 36 form four corners, one of which 38, defines an acute angle, two of which 40,42 define right angles, and one of which 44 defines an obtuse angle. In this situation, if a split vane such as 46 is formed on a wall adjacent an acute angled corner 38, the vane projects obliquely toward the adjacent cell 28, This has the effect of dissipating the swirl pattern that is desired around the given rod in the cell. This problem may generally be described as resulting from the difference, as viewed in plan, between the angle at which the vane projects from a given strip upper edge adjacent a corner, and the angle of the intersection of two strips at that corner.